Live, gold-dusted bacteria used in bioelectronic device

Oct 24, 2005

Ravi Saraf has gilded living creatures, but that's as far as his resemblance goes with Auric Goldfinger, the fictional villain in the 1964 James Bond movie.
The University of Nebraska-Lincoln chemical engineer used bacteria, not another human, and his goal was to explore electrical devices that could lead to important technological advances, not to take over the world.

Working with Vikas Berry, a doctoral student in his laboratory, Saraf created what he said he thinks is the first use of a microorganism to make a bioelectronic device with a live microorganism.

Saraf and Berry deposited bacteria (Bacillus cereus) on a standard silicon chip inlaid with gold electrodes. After the bacteria formed bridges between the electrodes, Saraf and Berry deposited gold nanoparticles measuring about 30 nanometers on the bacteria and introduced an electric current.

"On the bacteria's surface, there are these filaments that grab the nanoparticles," said Saraf, who came to UNL last year from Virginia Tech. "When the humidity increases, the bacteria swells because it absorbs moisture, and it contracts when the humidity goes down. When it swells or contracts, it increases or decreases the distance between the nanoparticles."

The distance between the particles, of course, affects their ability to exchange electrons and therefore their ability to pass on electrical current. Saraf and Berry found that a decrease of less than 0.2 nanometers between the gold nanoparticles (reflecting a decrease in humidity from 20 percent to essentially 0 percent), resulted in more than a 40-fold increase in electrical current.

"So now we have a very, very sensitive device that can measure humidity," Saraf said. "What is interesting is that the sensitivity of the device increases when the humidity goes down, which is completely opposite from other devices. Other devices work best when the humidity is high. They don't do well when the humidity is low. In the low-humidity range, our device is a factor of four to five times better than anything out there, in microelectronic devices."

The discovery was published by the highly respected German journal Angewandte Chemie International Edition. Funds from the Nebraska Tobacco Settlement Biomedical Research Development Fund helped supported the research effort.

Saraf said that if he lets his imagination go wild, he can envision this discovery leading to devices ideal for low-humidity, extraterrestrial environments in space and in high vacuums.

"That's great, but what really excites me is 'What's next?'" he said. "This work clearly shows that you can make nanodevices on live cells. Now, can we take the next step and have the live cell drive the nanodevice?"

Saraf said his idea is that microorganisms could be used to open and close electronic circuits, and maybe even power them.

"This is where I want to go and I actually have some reasons to believe it would work," he said. "If you can do that, now you can start thinking about a whole circuitry in which the microorganisms are driving the circuits and they're even powering it. You're powering it by giving microorganisms food. Instead of using batteries, which are caustic and environmentally unfriendly, you give it carbonaceous food, which is biodegradable. When you start thinking like that, you can add another level of logic that is based on nature's nanodevice, which is the microorganism. Nature is the best nanotechnologist. For example, the cell is a highly robust and sophisticated system of nanodevices. Therefore, combining it with physical nanodevice will add a high-level of functionality."

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